1. Field of the Invention
This invention relates in general to magnetic tunnel junction transducers for reading information signals from a magnetic medium and, in particular, to a magnetic tunnel junction sensor without current shunting, and to magnetic storage systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current I.sub.s from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
The use of an MTJ sensor as an MR read head requires a structure that generates an output signal that is both stable and linear with respect to the magnetic field strength from the recorded data on the disk. If some means is not used to maintain the ferromagnetic free layer of the MTJ sensor in a single magnetic domain state, the domain walls of magnetic domains will shift positions within the ferromagnetic free layer, causing noise which reduces the signal-to-noise ratio and which may give rise to an irreproducible response of the sensor. The problem of maintaining a single domain state is especially difficult in the case of an MTJ sensor because, unlike an GMR sensor, the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, and thus any metallic materials in contact with the edges of the ferromagnetic layers to produce a longitudinal magnetic bias field may provide a path for shunting the sensing current around the active region of the MTJ sensor resulting in reduced sensitivity.
An MTJ sensor having longitudinal biasing is disclosed in IBM's U.S. Pat. No. 5,729,410 ('410) granted to Fontana et al., and is incorporated in its entirety herein by reference. In the '410 patent, the biasing ferromagnetic layer is separated and electrically isolated from the edges of the MTJ sensor layers by an insulating layer having a thickness sufficient to electrically isolate the biasing layer from the MTJ sensor and the electrical leads but thin enough to permit magnetostatic coupling with the free layer. An insulating layer thickness in the range of 250-400 .ANG. separating the biasing layer from the MTJ sensor layer edges is needed to provide the required electrical isolation. This relatively large separation of the layers is a problem since the degree of magnetostatic coupling between the ferromagnetic biasing layer and the free layer decreases rapidly with increased separation of the layers especially when the separation of the layers becomes greater than the thickness of the layers. MTJ sensors having free layer thicknesses in the range of 20-50 .ANG. are needed to meet future requirements for high density magnetic storage. Separation of the ferromagnetic biasing layer from the free layer by more than five times the layer thicknesses will result in weak magnetostatic coupling resulting in decreased stability and reduced signal-to-noise of the MTJ sensor. In addition, the high sensitivity of magnetostatic coupling to the separation of the bias layer from the free layer will cause difficulty in controlling the degree of magnetostatic coupling, and therefore the hard bias properties during manufacturing.
What is needed is an MTJ sensor structure that provides the longitudinal magnetic biasing field required to provide a stable and linear response to the magnetic signal fields by reducing the separation of the ferromagnetic biasing layer from the free layer to values less than or in the range of the ferromagnetic layer thicknesses without providing a path for sensing current shunting around the active region of the MTJ sensor.